Servo-controlled actuator systems experience serious problems due to mechanical actuator resonances. These vibrational modes include the natural modes of the actuator and those of any intervening mechanical components. With increasing mechanical complexity, the vibrational modes of any given actuator system become difficult to predict. The problem is further compounded as the operating frequency of the actuator system is increased. The vibrational modes limit the control loop gain of the servo system, reduce bandwidth of the servo system, or both. This causes the controlled element, such as a transducer head, to experience excessive settling time after positioning, poor response to disturbances, poor tracking ability, or any combination of these.
Prior art systems have attempted to ensure stable operation of actuator systems by stabilizing the control loop. This has been done by inserting gain stabilizing filters such as electronic notch filters in the control loop path. These filters are placed in the downstream portion of the control loop to filter out the signal information within the band reject frequency range of the notch and thus help minimize excitation of these actuator vibrational modes.
The technique utilizing notch filters allows the servo control system to effectively ignore lightly damped structural actuator resonances. At the resonances very little control is applied by the servo controller.
The drawback to this technique is that it depends on the ability of the designer to accurately predict the frequency of the vibrational modes. This becomes increasingly difficult in high accuracy regimes because the servo system is exposed to many unforeseen disturbances that excite unanticipated vibrational modes. For example, in a hard drive actuator such disturbances include servo amplifier saturation and distortion, external forces on the arm assembly, e.g., due to seek activity, air turbulence, stiction and the like. Such disturbances are typically generated at points in the control path where correction is impossible when gain stabilizing filters are present in the control loop. Consequently, although notch filters are useful in reducing predicted resonances of the servo control system, they do not inhibit the excitation of other vibrational modes by agents external to the servo control loop.
Another technique for damping vibrational modes of a servo control system was presented by Masahito Kobayashi et al. in "MR-46 Carriage Acceleration Feedback Multi-Sensing Controller for Sector Servo Systems," at the International Conference on Micromechtronics for Information and Precision Equipment, Tokyo, Jul. 20-23, 1997. This proposed multi-sensing control system uses accelerometers to generate acceleration feedback. An acceleration feedback controller receives the feedback signals and compensates the servo to eliminate the mechanical resonance modes.
Although Kobayashi's technique has been demonstrated, it can not be efficiently implemented without the use of notch filters. Furthermore, designing the feedback controller requires the designer to model the very complicated transfer function H.sub.d (s) of the servo-controlled system. This is computationally challenging and requires a considerable amount of processing time. In addition, because the poles and zeros of the compensator used in the feedback controller can not be predetermined, it is not possible to guarantee the existence of a stable compensator.
The prior art also teaches gain stabilization through low-pass filtering in the control loop. In this approach the cutoff frequency of a low-pass filter that is inserted in the control loop is generally lower than the frequencies of any of the lightly damped resonances of the actuator structure. Thus, the components of the control signal having the resonance frequency are effectively prevented from exciting the vibrational modes of the actuator structure. This helps ensure system stability, but it also increases the phase shift at frequencies in the vicinity of the servo loop's unity gain crossing, thereby reducing the bandwidth of the servo system. In fact, this drawback applies to all gain stabilizing filters, including notch filters. The reduction in bandwidth, in turn, reduces the ability of the servo system to correct low frequency vibration and tracking performance such as run out and other disturbances that are due to external excitation and non-linearities in positioning operations.
In U.S. Pat. No. 5,459,383 Sidman et al. teach a feedback loop using a motion sensor located in the servo system at or near the point of control. The sensor is referred to as collocated because it is at or near the point of control. During operation the sensor detects both the rigid body motion and deformation of the actuator. The signal component from the rigid body motion is always much larger than that due to deformation. The collocated feedback loop operates in conjunction with the ordinary feedback loop and has the effect of making the servo system perform as if the mechanical structure of the system had a much higher mechanical damping than it actually possesses.
Although Sidman's system does improve positioning control and tolerance to internally and externally induced vibrational modes, it still relies on gain filters. Some negative effects of these filters are mitigated by the collocated feedback loop, but the most serious drawbacks, especially the requirement that the engineer know the vibrational modes ahead of time to ensure proper system design, are not obviated. Furthermore, the signal derived from the sensor includes the large rigid body component, which is also processed by the feedback loop and affects the rigid body motion properties of the actuator. This is undesirable since the rigid body properties of the actuator should be preserved.
Thus, the problem of stabilizing servo-controlled actuator systems remains. Solutions using filtering techniques are inadequate in high-accuracy regimes, e.g., in high density hard disk drives, since they require a priori knowledge of the vibrational modes of the system. Meanwhile, solving the transfer function to determine the vibrational modes is computationally unfeasible or impossible in most practical cases.
Finally, prior art systems suffer from the limitation of not being able to actively compensate for multiple vibrational modes at the same time. Specifically, if more than one single mode is selected for active control system stability is at risk.